Gardens are feeling the heat of summer.

Before this week’s rain, most of us had been watering to keep lawns and ornamentals alive. For best results, experts tell us, water deeply and early in the day.

There is a simple scientific reason behind this advice, says Mia Amato, author of “The Garden Explored,” an Accidental Scientist publication created at the Exploratorium, San Francisco’s museum of science, art and human perception. The book is part of a series designed to help readers discover that science is part of the things they do every day.

Plant cells hold a significant amount of water, but it varies from plant to plant. At its molecular level, water has the ability to dissolve minerals necessary for plant growth and to carry along critical gases such as oxygen and carbon dioxide. Roots absorb water that travels up the stem, and excess water evaporates back into the air through stomata, microscopic air vents on the surface of the leaves, in a process called transpiration.

The water movement through the plant is initiated not by the roots, but at the leaf level. As transpiration begins in response to increased sunlight, a simple suction pulls water upward throughout the day, from the roots into the leaves, Amato explained.

During hot, dry periods, water evaporation through the stomata causes the roots and stems to suck up all available soil moisture until the plant’s insides collapse and it wilts. In most plants, transpiration begins at sunrise and increases throughout the day.

By watering a plant in the early morning, as the stomata are beginning to open, transpiration will draw water up into the plant and the plant’s internal structure will efficiently distribute the water. The stomata close up late in the day as the sun sets. If a plant is watered in the evening, as the darkening sky signals the stomata to close, the water will stay around the root level and drain uselessly into the soil. Wet leaves and stems from watering late in the day may not be able to dry out adequately and often invite fungal diseases, such as downy mildew and rust.

A plant’s water requirement depends on how much it evaporates water, more on a sunny day than a cloudy one, and its location in a yard. A plant that gets dappled shade doesn’t need quite as much water as a plant that gets full sun all day.

Plants tell us when they need watering. Dull, limp or wilting leaves are generally a good indication. Some plants, such as hydrangeas, pineapple sage and Mexican bush sage, wilt faster than others. They can be used as indicator plants to signal that the rest of a garden needs water.

“Learning to conserve clean water should be part of every gardener’s life,” Amato aid.

She has an automatic timer on her sprinkler system, which turns on at sunrise. Drip irrigation systems are highly efficient, as well, because they deliver moisture directly to the root zones.

The benefit of mulching should never be underestimated. Three to four inches of mulch around plants will prevent weeds that rob moisture. Mulch also decreases the rate that water evaporates from the soil. Mulch should be applied early in the season when the soil is still moist and cool and should not touch the trunks of trees or woody shrubs, as it can hold too much moisture after heavy rains and promote disease.

The Old Farmers Almanac forecasts that the hottest period this summer for Vicksburg and surrounding areas is late June and early to mid-July, with a hurricane occurring in late July. September and October should be much cooler and slightly rainier than normal.

Water wisely this summer, and consider using more native plants and drought-resistant alternatives in the future.

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Miriam Jabour, a Master Gardener and Master Flower Show judge, has been active in the Openwood Plantation Garden Club for over 35 years. Write to her at 1114 Windy Lake Drive, Vicksburg, MS 39183.